1. Technical Field
The present invention relates to a method for servicing a wireless communications network in general, and in particular to a method for servicing a CDMA cellular/PCS telephone communications network. Still more particularly, the present invention relates to a method for extending hard-handoff boundaries of a boundary cell within a cellular/PCS telephone communications network.
2. Description of the Prior Art
For some code-division multiplex access (CDMA) cellular/Personal Communication Service (PCS) telephone communications networks, a single channel frequency is typically sufficient to provide service over the entire service area. However, in areas where the need for service is high, two or more channel frequencies are required to meet the demands. Within a dual channel frequency service area, if the second channel frequency is also covering the entire service area, theoretically a CDMA mobile can remain on one of the two channel frequencies while it is being moved anywhere within the service area. But this may not be true in practice because the second channel frequency is intended for handling heavy traffic areas such that the mobile has to switch its channel frequency while it is being moved across certain designated boundaries. The process of a CDMA mobile breaking from one base station, switching to a new channel frequency, and establishing a link with a new base station is known as a "hard-handoff."
There are typically two hard-handoff situations when a CDMA mobile operating at frequency f.sub.1 needs to be handed off to another frequency f.sub.2. The first situation is a "same-cell situation" in which the first carrier frequency f.sub.1 is ubiquitous within a coverage area while the second carrier frequency f.sub.2 is deployed to cover a limited area (such as a heavy traffic area) within the coverage area of first carrier frequency f.sub.1. The second situation occurs at the boundaries between two coverage areas, each area being separately covered by carrier frequencies f.sub.1 and f.sub.2. In the former case, the service provider is the same, while in the latter case the service provider can be either the same or different.
Referring now to the drawings and in particular to FIG. 1, there is illustrated a same-cell situation in which a second carrier is deployed in a limited number of cells within a coverage area of a first carrier. Assume that f.sub.1 is the first carrier frequency utilized throughout an entire coverage area 10 and that f.sub.2 is the second carrier frequency utilized to cover only a small number of cells within a heavy traffic region 11 (shown as shaded area in FIG. 1) of coverage area 10. There are three conditions that must be established in order to trigger a hard-handoff process: (1) the cellular telephone must be in an f.sub.2 boundary cell, i.e., a cell beyond which no service will be provided at frequency f.sub.2 ; (2) the mobile reports that all non-boundary sector pilot powers have fallen below a T-drop (a set value below which the mobile will drop the link to that cell); and (3) Round Trip Delay (RTD), i.e., a method of measuring the distance between the mobile and the base station, must be greater than a predetermined value.
If a CDMA mobile is near a cell boundary, then it is ready to be switched from frequency f.sub.2 to frequency f.sub.1. A RTD measurement is typically utilized to locate the position of the CDMA mobile,.and such information can be further utilized to determine whether or not it is near a cell boundary. However, RTD measurements are usually not very accurate in terms of locating the exact position of the CDMA mobile. Several conditions may limit the accuracy of RTD measurements. For example, a selector begins to receive a RTD report only after the RTD of the mobile has changed by at least two chips. This translates to an error of approximately 244 meters in establishing the exact location of the mobile. Further, the hard-handoff process may take up to five seconds. Now if the mobile is being moved at a speed of 70 m.p.h., then the mobile will have travelled at least 156 meters from the moment the hard-handoff process was initiated until the process is completed. Adding that to the RTD uncertainty (i.e., 244 meters), the mobile is almost 400 meters away from the cell boundary when the hard-handoff process is triggered. Because this f.sub.2 cell is a boundary cell, there is no soft-handoff gain for a cellular telephone that is going to switch to frequency f.sub.1, which results in a higher forward link E.sub.b /N.sub.o (energy/noise) requirement. This number has been estimated to be approximately 5 dB, which means that the reception level at the boundary cell is roughly 5 dB lower than the neighboring cells. In fact, the cells utilized in heavy urban areas are generally small already (typically around 1 kilometer); by shrinking f.sub.2 cells 5 dB in order to allow for the high E.sub.b /N.sub.o requirement, the f.sub.2 cells will be even smaller. As a result, the requirements of 400-meter distance from the cell edge and a 5 dB cell shrinkage may cause problems for cells in heavy traffic urban areas. This "shrunk" hard-handoff boundary 12 as shown in FIG. 1 indicates a significant reduction in the coverage area of the boundary cells.
With reference now to FIG. 2, there is illustrated a situation in which two different carriers are respectively deployed in two separate coverage areas that are adjacent to each other within a CDMA cellular/PCS telephone communications network. Assume that f.sub.1 is the carrier frequency for cells 21 on the left side and f.sub.2 is the carrier frequency for cells 22 on the right side. In this situation, there are three conditions that must be established in order to trigger a hard-handoff process: (1) the mobile must be in a boundary cell, which is a bi-sector cell in which service is provided by both frequencies f.sub.1 and f.sub.2 ; (2) the mobile reports that all non-boundary sector pilot powers have fallen below a T-drop; and (3) RTD must be greater than a predetermined value. These three conditions are similar to those of the same-cell situation, but here, overlayered bi-sector cells 23 (shown as a shaded area in FIG. 2) are operating under both frequencies f.sub.1 and f.sub.2. Bi-sector cells 23 are required to eliminate any ambiguity in determining the correct frequency of operation for the CDMA mobile and the base station to which it is going to be handed off. Note that in this case the mobile will be handed off to a co-located base station (i.e., a bi-sectored base station). The issues in this situation are similar to those of the same-cell deployment situation. As a result, the cell boundaries on both sides shrink, due to high E.sub.b /N.sub.o requirement, approximately 400 meters from the corresponding actual cell boundary. A "shrunk" hard-handoff boundary 24 indicates where the mobile switches from frequency f.sub.2 to frequency f.sub.1 and a "shrunk" hard-handoff boundary 25 indicates where the mobile switches from frequency f.sub.1 to frequency f.sub.2.
Consequently, it would be desirable to provide an improved method to extend the boundary of a boundary cell within a CDMA cellular/PCS telephone communications network such that the shrinkage in service area of a second carrier frequency can be reduced.